1. Field of the Invention
This invention relates to a fabrication method for creating patterned domain reversals in ferroelectric materials. The method can be applied to any ferroelectric material.
2. Description of the Related Art
Ferroelectric materials have internal electric dipole moments which can be made to manifest themselves on a macroscopic domain scale without the presence of external electric fields (hence the term “ferroelectric” by analogy with “ferromagnetic” for materials with domain scale magnetic dipole moments). These macroscopic polarizations are responsible for the optical properties of the materials through the important effects they have on the propagation of electromagnetic radiation. When the polarization of such materials is linearly dependent (or only very weakly non-linearly dependent) on the electric field strength of an electromagnetic wave propagating through the material, the effect of the linear polarization is to produce a constant refractive index, which is responsible for modifying the speed of the wave through the material. In such a linear case, an incident oscillating electromagnetic field at frequency ω produces an oscillation of the polarization at the same frequency co which, in turn, produces a re-radiated electromagnetic field also of the same frequency but out of phase with the incident wave. The original incident wave, combined with the phase-varying re-radiated waves along the forward propagation direction of the incident wave, creates a net transmitted wave that moves through the material at an apparently slower speed but same frequency. The speed, v(ω), of the transmitted wave in the crystal, is defined as c/n(ω), where c is the speed of the wave in vacuum (ie., the speed of light) and n(ω) is the index of refraction of the medium which, as indicated, depends on the frequency ω of the wave (ie., the medium is generally dispersive). Another important parameter of the medium is k(ω), the propagation constant of the radiation, which is defined as: k(ω)=2πn(ω)/λ, where λ is the wavelength of the wave in vacuum.
When the polarization of the crystal is made to change by the imposition of an external electric field that is not the oscillating field of the incident electromagnetic wave, then interactions between the wave and the material can occur which are not simply describable by a constant index of refraction that simply changes the wave speed. For example, the so-called electrooptic effect results when the application of a constant electric field is used to rotate the dipole moment directions of a crystal and to thereby change both the speed and the polarization direction of an incident wave.
If the polarization at a position x within the crystal is a non-linear function of the field at that position, the propagation of an electromagnetic wave can be affected in additional ways. For example, the propagation of a wave with frequency ω1 will lead to the propagation of a secondary wave with frequency 2ω1, which is the second harmonic of the wave. If two waves, of different frequencies ω1 and ω2 simultaneously propagate through the non-linear crystal, there will be generated additional waves of frequencies ω1+ω2, ω1−ω2, 2ω1 and 2ω2. Each wave periodically modulates the polarization which the other wave sees and which it sees itself and, as a result, new waves are generated.
When waves at two different frequencies ω1 and ω2 and two different propagation constants k1 and k2 propagate a distance L through a non-linear crystal, one wave (k1) can transfer power to the other wave (k2) through the non-linearity of the polarization. The amount of power transferred after the wave has traveled a distance L in the crystal can be shown to be proportional to: L2(sin x/x)2, where x=(k3−k1−k2)L/2=LΔk/2 and k3 is the propagation constant of the polarization wave. If Δk is not zero, the transfer of power reaches a maximum value when the wave has propagated a distance called the coherence length, Lc, where LcΔk/2=π/2, (ie. Lc=π/Δk). If Δk=0, the incident waves and the polarization wave are said to be phase matched and the power transfer increases along the entire length of the crystal and is proportional to L2, where L is the length of the entire crystal. If Δk is not zero, the maximum power transfer occurs within the coherence length, then goes to zero, then rises again in the next coherence length. In general, the power transfer within the coherence length is the maximum transfer possible, no matter through how many coherence lengths the waves propagate. Since obtaining a phase matched condition is very difficult in practice (it has been obtained using birefringent crystals) and will only occur at particular k values, an alternative approach to maximizing power transfer is through “quasi-phase matching” or QPM. QPM is obtained by changing the phase between the propagating wave and the polarization wave by π/2 every time the propagating wave moves through an additional coherence length. This can be accomplished by rotating the polarization direction within the crystal by 180° in successive coherence lengths. Unlike true phase matching, QPM can be obtained for a wave of arbitrary k value, providing the polarizations in the crystal can be rendered appropriately periodic in successive coherence lengths. Thus, by satisfying the QPM condition, the maximum power transfer is incremented in each successive passage of the wave through a coherence length, rather than falling to zero. Such a periodic rotation of polarization domains (domain reversals) of width Lc is usually accomplished by the application of electric fields, electron beam patterning or proton exchange/heat treatment (of which more will be said in the following) and the process is called “poling.” Although quasi-phase matching does not produce the amount of power transfer produced by genuine phase matching (because the factor (sin x/x)2 is 1 for Δk=0, but is 4/π2 for Lc Δk=π/2), it is much better than the non-QPM case. Much inventive effort has been expended in finding ways of rotating crystal polarizations in a periodic manner with the coherence length being the period.
A case of particular interest in modern technology occurs when ω2=2ω1, which is called frequency doubling or second harmonic generation (SHG). Obtaining a meaningful power transfer between an incident wave and its frequency doubled second harmonic allows the production, for example, of coherent green or blue light by the passage of near infra-red radiation from a solid state laser through a non-linear ferroelectric crystal. Since coherent infra-red radiation is easier to produce by laser action than coherent blue or green radiation, being able to obtain the green or blue by SHG is quite important. Such green or blue light is important for reading and writing optical storage disks. The coherence length needed to obtain efficient frequency doubling is given by: Lc=π/(2k1−k2). Note that 2k1−k2 is not zero because of the dispersion of the material, so true phase matching is generally not possible. As noted above, use of QPL by poling non-linear ferroelectric crystals such as congruent lithium niobate (LiNbO3, or CLN) or stoichiometric lithium tantalate (LiTaO3 or SLT) allows frequency doubling of radiation within the entire range of frequencies for which these crystals are transparent: (0.32 microns–5.5 microns) for CLN and (0.27 microns–5.5 microns) for SLT. The following prior art teaches a variety of methods for patterning and reversing polarization domains to achieve QLM in non-linear ferroelectric materials.
Nihei et al. (U.S. Pat. No. 5,424,867), teaches a method for fabricating an optical wavelength converter with a high threshold for optical damage. It is pointed out that a crystal may be damaged by a second harmonic wave having a relatively low power of 2 mW at a wavelength of 477 nm. Within the method, periodic domain reversals are formed on a LiNbO3 substrate which is covered by a thin, transparent conducting film of indium-tin-oxide (ITO). The film then dissipates surface charges formed by the polarization process without, at the same time, producing reflection or scattering which would degrade the signal. Within the method domain reversals within the crystal substrate are accomplished by an electron beam having an energy between 20–30 kV. The period of the domain reversals is set to 4.7 microns which allows a first order period of 946 nm to produce a second order harmonic of period 477 nm.
Yamada et al. (U.S. Pat. No. 5,249,250) notes that the depth of a domain reversal and the width of a domain reversal (pitch) are related so that a narrow pitch (high frequency) results in a shallow depth. A shallow depth reduces the region of the crystal through which the SHG will occur. To solve this problem, Yamada teaches the formation of domain reversed regions by first polarizing the crystal uni-directionally (a single domain), then irradiating the crystal surface with a 15 kV electron beam in a pattern of regularly spaced parallel strips. An important aspect of this method is that the inverted domain structure can be formed without adversely affecting the index of refraction of the crystal.
Nozaki et al. (U.S. Pat. No. 5,395,495) teaches a method of forming domain reversals within a ferroelectric crystal wherein a high resistance layer is first formed on a uni-directionally polarized crystal surface and a charged particle beam is then directed into the crystal through the layer. This method alleviates the problem of forming effective domain reversals in the vicinity of the crystal surface.
Harada et al. (U.S. Pat. No. 5,415,743) teaches a method of forming sharply defined domain reversals that extend through the entire thickness of the ferroelectric crystal. The method teaches the formation of proton-exchanged regions on a unipolarized crystal and then heating the regions by the application of external electric fields. Also taught in the method is the formation of a Ti-diffused region which is also heated by an electric field, forming ion-implanted regions which are heated by an electric field and irradiating selected regions with light, followed by subsequent field treatment. The electric field in each of these methods is provided by a corona discharge.
Harada et al. (U.S. Pat. No. 5,568,308) teaches the formation of domain reversals in a MgO-LN non-linear unipolarized ferroelectric crystal by first proton-exchanging an appropriate region and then applying patterned electrodes to a surface of the crystal and creating an electric field between the electrodes. The electrodes consist of a separated pair formed on an upper surface of the crystal wherein one of the pair is comb-shaped and the other of the pair is rectangular. In an alternative embodiment, the field is provided by a corona wire.
Harada et al. (U.S. Pat. No. 5,570,225) teaches the formation of domain reversals in a MgO—LiNbO3 or a MgO—LiTaO3 non-linear unipolarized ferroelectric crystal by first proton-exchanging an appropriate region, diffusing Ti through the region or diffusing Li through the region and then applying electrodes to a surface of the crystal and creating an electric field by applying a direct or pulsed current to the electrodes.
Harada et al. (U.S. Pat. No. 5,522,973) teaches the formation of domain reversals in a MgO—LiNbO3 or a MgO—LiTaO3 non-linear unipolarized ferroelectric crystal by first proton-exchanging an appropriate region and then applying patterned electrodes to a surface of the crystal and creating an electric field between the electrodes. The electrodes consist of a separated pair formed on an upper surface of the crystal wherein one of the pair is comb-shaped and the other of the pair is rectangular. In an alternative embodiment, the field is provided by a corona wire.
Harada et al. (U.S. Pat. No. 5,594,746) teach a method for forming domain reversals in a ferroelectric crystal using a corona wire and a pattern of electrodes formed on the crystal. There is also taught an apparatus for creating the domain reversals comprising the electrode, the corona wire, a power source and an evacuated chamber.
Byer et al. (U.S. Pat. No. 6,156,255) teach a method for forming patterned domain reversals in a non-linear ferroelectric material using an electric field applied by means of spaced conductors. There is also taught a method of first characterizing the material to determine the most efficient way to achieve the desired result. The characterization seeks to establish the proper application of surface treatments as well as the best geometry of the crystal.
The methods disclosed above produce some disadvantageous effects. In particular, Ti indiffusion, Li outdiffusion and proton exchange followed by heat treatment, produces undesirable changes in the index of refraction of the material. The use of corona discharge methods and the use of high temperature thermal cycles and vacuum processing increase fabrication complexity and expense. In addition, such processes can damage crystal surfaces and, thereby, adversely affect the refractive properties of the crystal. The disavantages of the various methods can be summarized as follows:
(1) Many of the methods cited above produce high temperatures which cause low throughput and process control difficulties. A ferroelectric material should be heated to the Curie temperature to reduce the magnitude of the external field required to create domain reversals in all methods except the E-field method (electric field between applied electrodes) and the corona discharge method. Thus, the average operating temperature is approximately 500° C., which can cause wafer breakdowns, lead to stresses in the heating and cooling cycles and also result in the diffusion of metals into the ferroelectric material.
(2) The necessity of high vacuum processing slows down the process cycle and reduces throughput. Domain reversal techniques based on electron discharge require high vacuum to prevent interference from the environment and arcing between discrete electrodes. A vacuum of the order of 10−7 Torr takes a long time to produce.
(3) The depth of the domain reversal is often too shallow to use a bulk device like a solid state laser chip. Most domain reversal processes use surface chemical reactions, such as Ti in-diffusion, Li2O out-diffusion or proton exchange to lower the Curie temperature. In these cases, the depth of domain inversion is on the order of several microns. It is, therefore, only suitable to use such domain-reversed crystals in conjunction with surface devices such as wave-guides or surface acoustic wave (SAW) devices. To use bulk devices as radiation sources would require a ferroelectric device with domain reversals to a depth of millimeters.
(4) The scanning rate of domain reversals is very slow in most methods. To obtain a high quality (sharply defined) domain reversal, requires the use of slow methods, such as electron-beam writing or corona discharge.
Based on the limitations described above, we conclude that the E-field method (electric field produced between electrodes deposited on the material) has more advantages than the other methods. The chief disadvantages of this method are seen when the domain reversals must have a short period (narrow width) of about 10 microns or less and the wafer thickness is about 0.5 mm in CLN. In this case, the domain walls do not retain a planarity between the upper and lower surface planes of the crystal (between the +z and the −z surfaces). Moreover, in thicker wafers, the domain wall planarity is even worse. To alleviate these problems, the present invention teaches a double-sided poling process for ferroelectric domain reversals which also includes a new alignment technique. The method facilitates well defined poling in large area wafers and yields uniform domain reversals with a good, controllable process duty cycle and high quality domain walls.